1. Field of the Invention
This invention relates in general to the casting of plastic optical components, and more particularly to replication processes for casting plastic lenses using replica molds formed from polymeric compositions which impart properties to the replica molds necessary to cast high-quality ophthalmic lenses.
2. Description of the Prior Art
Replication molding techniques are well known. In such a technique, a replica mold having a surface which is a negative copy of a surface of the final product is first prepared from a master mold. The final product is then molded against the negative surface of the replica mold, thus reproducing the surface configuration of the master mold.
Attempts have been made to manufacture plastic optical elements, particularly plastic ophthalmic lenses, by replication techniques, but satisfactory results have not heretofore been obtained. Exemplary of such attempts are the processes taught in U.S. Pat. Nos. 3,422,168 and 3,423,488, both to G. H. Bowser. These patents teach a replication process in which plastic lenses are cast from replica molds formed from materials such as low molecular weight, non-emulsifiable polyethylene resins. It has been found, however, that plastic lenses cast using Bowser type polyethylene replica molds invariably contain certain defects including minute surface imperfections which cannot be eliminated from the polyethylene replica mold surfaces. Some of these surface imperfections include scratch-like markings, surface blemishes, graying, orange peel, etc. Additionally, Bowser-type polyethylene replica molds have high degrees of flexibility, and because of this, distortion of the plastic lens occurs during cure which makes it impossible to reproduce consistent lens powers from one casting to the next with the accuracy required by ophthalmic standards. Lens distortion is a particular problem when polyethylene replica molds are used to cast high power lenses, but even plano (zero power) lenses cannot be cast to any degree of repeatable accuracy with these materials.
The criticality of the polymeric compositions used to form replica molds has been confirmed by many unsuccessful attempts to obtain accurate replications of glass surfaces using a multitude of commercially available resins to form the replica molds. It has been found that the great majority of these resins do not work satisfactorily for one reason or another. Some resins tried unsuccessfully include polymers of allyl methacrylate, styrene triallyl cyanurate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, and even polymerized allyl diglycol carbonate itself as a replica mold material.
It is believed that there are many reasons why most polymeric compositions tried do not yield satisfactory results in replication processes such as those described herein. For example, low-melting polyethylenes, such as those taught by Bowser, appear to form microscopic defects in the mold surfaces which reproduce in the final plastic lenses. Also, these materials are not believed to be sufficiently rigid and stiff to prevent distortion of the curing plastic lens. On the other hand, materials such as linear polymethyl methacrylate produce replica molds whose surfaces are solvent attacked by one of the materials primarily used in forming plastic lenses, i.e. allyl diglycol carbonate resin (sold commercially by Pittsburgh Plate Glass Co. under the registered trademark CR-39). A solvent attack of this nature produces a matte finish on the final plastic lens making it unsatisfactory.
Another property which makes some polymeric materials unacceptable is their release characteristics from the plastic lens being molded. Examples of materials which do not have satisfactory release characteristics from cured CR-39 include such resins as linear polystyrene, polyester, alkyd and epoxy.
Despite the fact that the attempts to date have been unsuccessful in arriving at a replication process using polymeric replica molds which produces high-quality optical components and which is commercially feasible, there is a tremendous amount of research continuing in this area for such a process. The many great advantages costwise and otherwise which would flow from such a successful process are well documented in the literature in general and in particular in the above-mentioned Bowser patents.